The present invention relates primarily to multicell storage batteries which are normally sealed in a gas-tight manner.
In gas-tight batteries, overcharging or charging with too high a current, as well as reversals in polarity, can produce high internal gas pressures (i.e., gassing) which can ultimately lead to cell bursting. For this reason, steps are conventionally taken to control such excess pressures and their related safety risks. The smaller batteries can be provided with weakened areas, forming "nominal failure points" on the housing. The larger batteries can be provided with pressure relief valves or rupture disks for achieving the desired result.
A preferred field of application for the improvements of the present invention is in prismatic nickel/hydride cells. This especially pertains to batteries having a plurality of cells, in particular, those developed as power sources for electric-powered street vehicles (i.e., traction batteries). Prismatic nickel/hydride cells have the advantage over round cells of better economy of space, but often lack the necessary stability in terms of their shape in the event of excessive gas pressures under extreme operating conditions. Prevention of the deformation of the cell housing by the timely release of gas, or the bursting of a rupture disk, is especially important in such cases.
In the absence of corrective measures, bulging usually accompanies disturbances in the cells such as heating and voltage deviation (downward in the case of a load, upward in the case of a charge, etc.). At times, a dangerous state is reached, which can lead to highly progressive damage. This is especially so in cases where the gassing and/or bulging is not noticed and operation of the (essentially dried-out) cell is continued. In such case, a cell may come to be filled with an ignitable gas mixture, and may overheat because the electrolyte level is too low or due to failure of the separator. This can lead to a short circuit, which may then trigger an ignition. The result is an explosion, destruction of the battery, and potential safety concerns.
As a rule, this result is preventable by appropriate safety measures, as previously noted. However, such measures can themselves lead to certain deficiencies. For example, while pressure relief valves assure a reclosure of the cell, and accordingly, a lengthening of the cell's operating life, no clear signal is given to indicate damage to the cell. The flow resistance of the open valve is at time so great that the available opening (cross-section) is insufficient to handle the blow-off of collected gases. In the case of a strong overcharging (e.g., with 10.times.I.sub.5 to 20.times.I.sub.5) and an unlimited voltage, the pressure in the cell can rise so fast that the generated gases cannot flow out through the blow-off valve rapidly enough. The result is that the cell will burst.
In such cases, replacement of the valve with a more simply constructed, lighter and more economical rupture disk is recommended. This is because even a correctly functioning valve will not always prevent slow (progressive) damage to the cell by releasing gas. A rupture disk will burst so that damage to the cell is made obvious and the cell can be immediately replaced. However, in either case (i.e., when a valve is used and also when a rupture disk is used), it is nevertheless important that a loss of gas be indicated in some way.
German Utility Patent No. 67 51 613 discloses a pressure safety release capable of direct visual monitoring. To this end, a burstable membrane is provided which can be destroyed by a bursting needle. In addition, a colored (and therefore more easily observed) visible membrane is provided to protect against the outer atmosphere. However, the visible membrane has a bursting pressure which is lower than the response pressure of the bursting membrane. As a result, if the bursting membrane is destroyed, this results in an immediate load upon and destruction of the visual membrane. An immediate response to such an event would require that the maintenance person always be near and attentive.